The present invention relates to a semiconductor laser device, and more particularly to a tunable semiconductor laser device which can vary the wavelength of generated light in a predetermined range. In other words, the present invention relates to a semiconductor laser device which can be used as a transmitting light source or optical local oscillator in coherent optical communication and high bit-rate optical communication.
A semiconductor laser of the variable wavelength type is described in, for example, Extended Abstract No. 29a-T-7 (The 47-th Autumn Meeting, 1986) the Japan Society of Applied Physics. FIG. 2 shows the structural outline of this semiconductor laser, and is a sectional view taken along the axial direction of a resonant cavity. Referring to FIG. 2, an active region I and a DBR (distributed Bragg reflector) region II are formed on a semiconductor substrate 201. A diffraction grating 211 is formed on that surface area of the substrate 201 which corresponds to the DBR region II, and an optical guide layer 202 and a buffer layer 203 are successively piled all over the surface. Further, semiconductor layers including a cladding layer 205 are piled, and then a current injecting electrode 207 (208) and an opposite electrode 209 are formed. Thereafter, a groove 210 is cut to separate the active region I electrically from the DBR region II. Accordingly, the current injecting electrode is separated into an electrode 208 corresponding to the active region I and an electrode 207 corresponding to the DBR region II. Thus, an exciting current for generating light in an active layer 204 is supplied from the electrode 208 into the active region I, and a current for varying the wavelength of generated light is supplied from the electrode 207 into the DBR region. The current supplied to the electrode 207, that is, carriers injected into the DBR region II vary the refractive index of the optical guide layer 202 included in the DBR region II. This is based upon a phenomenon that the refractive index of the optical guide layer 202 made of a semiconductor material varies in accordance with the carrier density in the layer 202, that is, a plasma effect. Such a change in refractive index of the optical guide layer 202 causes a change in grating constant (namely, perturbation period) of the diffraction grating 211. Thus, when light generated in the active region I is propagated into the DBR region II, the light is affected by the change in perturbation period of the grating 211, and hence the wavelength of fed-back light is varied. Thus, the wavelength of the laser beam emitted from the semiconductor laser can be varied. That is, even when the current supplied to the active region I is kept constant, the wavelength of the laser beam can be varied by changing the current supplied to the DBR regions (that is, the amount of carriers injected into the DBR region).
In tunable semiconductor lasers including the above laser, however, there arises a problem that the wavelength of generated light can be varied only in a narrow range. In more detail, the wavelength of generated light can be varied by several nanometers at most. A tunable semiconductor laser having such a narrow wavelength range is not useful from a practical point of view. Thus, it has been required to develop a tunable semiconductor laser in which the wavelength of generated light can be varied by at least tens of nanometers. However, such a semiconductor laser has not yet been developed. This is because in the prior art, only the carrier density for controlling the refractive index of the optical guide layer 202 included in the DBR region II (that is, the carrier density for controlling the period of optical perturbation in the DBR region II) is used for varying the wavelength of generated light, that is, the wavelength of generated light is varied only by one parameter, and thus it is impossible to enlarge the wavelength range sufficiently.